FIGS. 1A and 1B are examples of a touch input circuit for describing a principle of self capacitive touch input. The touch input circuit may be an component of a electronic device.
To a node n1, {circle around (1)} an ‘equivalent capacitance’ formed by a capacitance Cf 18, a parasite capacitance Cp 20, and other capacitance Ce 23, {circle around (2)} a resistor Rref 12, {circle around (3)} a non-inversion input terminal of an operation amplifier 15, {circle around (4)} a switch 14, and {circle around (5)} an electrode pad 16 may be connected. The electrode pad 16 may be a transparent or opaque conductive material. A reference potential Vref may be supplied to an inversion input terminal (−) of the operation amplifier 15. In one exemplary embodiment, the reference potential Vref may be larger than a ground potential.
In this case, when a conductor such as a finger is close to the electrode pad 16 when voltage is applied to the electrode pad 16, the capacitance Cf 18 as an element configuring the ‘equivalent capacitance’ is generated by forming an electric field between the capacitance Cf 18 and the conductor. That is, a value of the capacitance Cf 18 is changed according to whether a touch is input, and whether the touch is input may be verified by measuring the changed value.
Meanwhile, the parasite capacitance Cp 20 may be a capacitance which is not intently designed and formed between other parts of the electronic device and the electrode pad 16. Accordingly, the value of the parasite capacitance Cp 20 may be a value which may not be known in advance by the designer of the touch input circuit.
In this case, only when the value of the parasite capacitance Cp 20 is sufficiently small or not present, a change amount of the capacitance Cf 18 may be easily measured.
Further, noise which is generated or input from other parts of the electronic device is transferred to the node n1 through a node n2 which is present at one end of the parasite capacitance Cp. Other capacitance Ce 23 formed at another part of the electronic device may be further connected to the node n2.
An on/off state of the switch 14 may be determined on the basis of a difference value between reference voltage Vref applied to the inversion input terminal (−) of the operation amplifier 15 and voltage Vx of the node n1 applied to the non-inversion input terminal (+).
As illustrated in FIG. 1B, the voltage Vx of the node n1 may vary according to a change of the on/off state of the switch 14. ‘Off’ disclosed in a lateral axis of FIG. 1B means a time period for which the switch 14 maintains the off state, and ‘On’ may mean a time period for which the switch 14 maintains the on state.
When the switch 14 is in the on state, a change rate of the voltage Vx may be determined by a time constant τ which is determined by the ‘equivalent capacitance’ and the resistor Rref 12. When the switch 14 is in the off state, the voltage Vx drops to the reference potential again.
According to how much a finger 17 is close to the electrode pad 16, the magnitude of the capacitance Cf 18 may be changed, and as a result, the magnitude of the ‘equivalent capacitance’ may be changed. Accordingly, the value of the time constant τ may be changed according to the change amount of the capacitance Cf. The change of the time constant τ influences the change rate of the voltage Vx in the time period when the switch 14 maintains the on state as illustrated in FIG. 1B. Accordingly, information regarding the size of the time constant τ, the magnitude of the capacitance Cf 18, and how much the finger 17 influences the electrode pad 16 may be reversely calculated by using a value for a voltage Vx graph. As a result, it may be determined whether the touch input is performed.
For example, when the finger 17 is not present around the electrode pad 16, the capacitance Cf 18 is not present, and as a result, it may be assumed that the value of the ‘equivalent capacitance’ is Ce1. Thus, when the finger 17 is present around the electrode pad 16, the capacitance Cf 18 is present, and as a result, when the value of the equivalent capacitance’ is Ce2, a relationship of Ce2>Ce1 may be satisfied. As a result, a time constant tau1 when the finger 17 is not present around the electrode pad 16 is smaller than a time constant tau2 when the finger 16 is present around the electrode pad 16. In FIG. 1B, when the finger 17 is not present around the electrode pad 16, the voltage Vx may more rapidly increased as compared with the finger 16 is present around the electrode pad 16. When using the phenomenon, for example, it may be determined whether the finger 16 is present around the electrode pad 16 by measuring a time taken for the voltage Vx to increase from 0 to Vref.
FIGS. 1C and 1D illustrate a circuit in which the resistor Rref 12 of FIG. 1A is replaced with a constant current source Iref 12_1 as the circuits corresponding to FIGS. 1A and 1B and a change according to a time of the voltage Vx at this time. An operation of the circuit according to FIGS. 1C and 1D can be easily understood by a person who understands the operational principle described in FIGS. 1A and 1B.
FIGS. 1E and 1F are examples of a touch input circuit for describing a principle of a mutual capacitive touch input. The touch input circuit may be a component of a electronic device.
Referring to FIG. 1E, a first electrode pad VCOM 11 and a second electrode pad VCOM 12 are insulated from each other by an insulator 511 on a substrate 512. In this case, when predetermined voltage is applied to the first electrode pad VCOM 11, a magnetic flux 510 generated in the first electrode pad VCOM 11 is directed to the second electrode pad VCOM 12. In this case, a mutual capacitance Cs is formed between the first electrode pad VCOM 11 and the second electrode pad VCOM 12 by the magnetic flux 510. In this case, when a touch input tool such as a finger is present in a space where the magnetic flux 510 discharged outside the insulator is included, the magnetic flux 510 discharged outside is not input to the second electrode pad VCOM 12. Accordingly, the value of the mutual capacitance Cs is changed. The mutual capacitive touch input circuit determines the touch input or not by measuring the value of the aforementioned mutual capacitance Cs. Like the first electrode pad VCOM 11 of FIG. 1E, an electrode to which the predetermined voltage is applied may be called a driving electrode pad and the second electrode pad VCOM 12 may be called a sensing electrode pad.
FIG. 1F illustrates an example of the mutual capacitive touch input circuit and an example of a switched capacitor integrated circuit. In FIG. 1F, two switches shift on/off states according to a first clock Clk1 and a second clock Clk2, respectively, and do not share the time period of the on-state. Current provided from a power source Vs(t) is charged in the capacitance Cs and then the charged charge is stored in an integral capacitance Cfb which is connected to the operation amplifier. That is, the capacitance Cs allows charges to be continuously accumulated at two ends of the integral capacitance Cfb while charge and discharge are continuously repeated. As the value of the capacitance Cs is increased, more charges per unit time may be charged at the two ends of the integral capacitance Cfb. Accordingly, the magnitude of the capacitance Cs may be determined by verifying output voltage Vfb(t) of the operation amplifier. In this case, the two ends of the capacitance Cs of FIG. 1F may be designed to be the first electrode pad VCOM 11 and the second electrode pad VCOM 12 of FIG. 1E, respectively.
The aforementioned electrode pads 16 are disposed vertically and horizontally in plural to measure the touch input or not for the electrode pad by the self capacitive type illustrated in FIGS. 1A to 1D. In this case, as the number of electrode pads 16 is increased, power consumption of the circuit for sensing the touch input is increased. Alternatively, as the number of operation amplifiers is increased, complexity of the circuit is increased. For example, when arrangement of the electrode pads has a matrix structure of 20*12, a total of 240 electrode pads are provided. When the aforementioned operation amplifier needs to be connected to each electrode pad one by one, the complexity of the circuit is very increased.